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In 2014, Black phosphorus (BP) was rediscovered as a 2D layered semiconductor with tunable band gap and respectable carrier mobility for next-generation electronics and optoelectronics applications. After years of intense study, one key remaining challenge harassing the whole community is how to get perfect phosphorene and what are the intrinsic properties of phosphorene.

To better answer this question, our efforts were turned to isolate phosphorene from bulk black phosphorus with minimized degradation, but retain its intrinsic properties. Due to the sensitivity of few-layer phosphorene in ambient conditions, most preliminary trial to isolate phosphorene results in failure due to fast oxidation during processing and device fabrication.

The innovation of currently presented strategy is to synthesize artificial monolayer atomic crystal molecular superlattices (MACMS) consisting of alternating layers of monolayer crystals and molecular layers through molecular intercalation. By selecting proper molecules and optimizing intercalation condition, monolayer phosphorene molecular superlattices (MPMS) with the interlayer distance up to 11.27 Å were visualized from TEM cross-sectional imaging and verified by XRD. What's surprising, in situ optical properties study and followed transportation study revealed, for the first time, an optical band gap (2.26 eV) matching up to and exceeding the theoretically predicted band gap value for monolayer phosphorene (~ 2eV) and stable FET devices from monolayer material with a best combination of carrier mobility of 328 cm2/V/s, on/off ratio up to 107, and environmental stability at 300 hour level, which presents the first demonstration of molecular intercalation in BP to produce a unique monolayer superlattice structure without compromising the electronic properties of the host materials.

The unique superlattice structure thus offers a new material platform for both the fundamental physics studies and next generation device applications of phosphorene, such as the topological transition under pressure due to Fermi phase and band structure change, compared with traditional Dirac semimetal behavior of black phosphorus, or the low temperature quantum transportation studies and its relationship with lattice symmetry. Additional studies suggested that our approach can be readily applied to diverse 2D materials (including MoS2, WSe2, SnSe, GeS, NbSe2, Bi2Se3, and In2Se3) to produce a wide array of 2D superlattices with tunable structural, chemical, electronic and optoelectronic properties.
With a large library of 2D materials and nearly “infinite” choices of organic intercalants with variable sizes, symmetries and functional substituents, our study defines a general route to a broad class of 2D superlattices, and enables a new dimension to tailor and tame the structural, physical, chemical, electronic, optical and magnetic properties of 2D atomic crystals. We believe our study will ignite a new wave of excitement on 2D materials and their molecular superlattices, and greatly promote the 2D materials research to a new chapter.

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